GBT News Update

6 May 2001

As reported in previous news updates, commissioning operations were suspended on April 4 so that Lockheed-Martin and NRAO could investigate a problem with the azimuth track.   The track wear plate segments and the base plate segments below it were shifting in a circumferential direction as the azimuth wheel trucks of the antenna passed over them.  A consequence of this movement was that, over time, the wear plate hold-down bolts were fracturing just below the head.  A consultant who visited the site in April felt that the wear plate segments could be secured against damaging movements by bolting them down with more, and larger diameter bolts.  Acting upon this advice, Lockheed-Martin tried a test in which the number of bolts in a segment was approximately tripled, and their diameter increased from 1 to 1.5 inches.  The azimuth wheels were then run across the segment and the movement was measured by NRAO engineers.  The measurements showed that the circumferential motion was reduced from as much as 3 mm (0.125 inches) to ~0.075 mm (0.003 inches).  The wheel trucks were run back and forth across the segment 10 times, and showed no degradation in performance from start to finish of the test.  This represented a considerable improvement and is deemed by NRAO and the contractor as an acceptable way forward to bring the GBT back on line.  A further detailed analysis of the track is also proceeding.  Lockheed-Martin has begun the work to put the additional, larger diameter bolts in all segments and expects to have this work completed by early July.  The NRAO expects the GBT to resume commissioning observations by mid-July.

In other work, the previously scheduled replacement of seven of the azimuth wheel bearings was begun on May 1.  In the first week, two of the seven wheel bearings had been replaced.  Lockheed-Martin expects this work to be completed on schedule, by the end of May.  The contactor is also using this time to proceed with painting of the structure.

P. R. Jewell

M. M. McKinnon